The present invention relates generally to a method of determining the inner diameter of a bore. Specifically, the present invention relates to a method of sizing and analyzing the inner diameter of a center bore of a rotor to determine the acceptability of the rotor for attachment to a crankshaft.
A squirrel cage rotor for use in an induction motor has a rotor core and a rotor cage with rotor bars that extend through the rotor core and end rings that connect the rotor bars together at each end of the rotor core. The rotor core is typically made of a magnetic material such as iron or steel and the rotor cage is typically made of an electrically conductive material such as copper, aluminum or an aluminum alloy. The rotor core has a substantially cylindrical shape with a longitudinally extending center or central bore to receive the shaft of the motor and a plurality of longitudinally extending rotor slots or apertures, which rotor slots may be slightly skewed, to receive corresponding rotor bars of the rotor cage. A laminated rotor core is commonly manufactured or formed by stacking or assembling a plurality of discs or laminations of the magnetic material on top of each other until the desired substantially cylindrical shape is obtained. During the stacking or assembling process, the laminations are also aligned or oriented into their proper position. Alternatively, the rotor core can be manufactured from a single piece of the magnetic material, but this technique is less common.
Each lamination in the rotor core is formed or extruded to a pre-selected thickness, shape and configuration. The pre-selected configuration of the laminations includes an aperture for the central bore, a plurality of apertures for the rotor slots positioned equidistantly about the central bore and a predetermined bridge thickness, which bridge thickness is defined as the radial distance between the outer circumference of the lamination and the aperture for the rotor slot. The pre-selected configuration of the lamination can also include other features as needed. As the laminations are stacked to form the rotor core, they are aligned and/or oriented into an appropriate position to form substantially continuous apertures in the rotor core and, if necessary, other desired features of the rotor core.
Next, the rotor cage is manufactured or formed by inserting, casting or injection molding a rotor bar into each of the plurality of rotor slots in the rotor core, which rotor bars extend to at least the ends of the rotor slots, and connecting the adjacent ends of the rotor bars to each other with an end ring. When the rotor is ready for attachment to the crankshaft or shaft of the compressor, which crankshaft generally has an outer diameter greater than the inner diameter of the center bore of the rotor, the rotor is heated to a suitable temperature, typically about 450° F., in a heat shrinking or shrink-fitting operation. This heating of the rotor expands the center bore of the rotor a sufficient amount to receive the crankshaft. After the crankshaft is inserted into the center bore, the rotor is cooled to contract around the crankshaft forming a tight interference fit between the rotor and the crankshaft.
One problem with using a laminated rotor is that the center bore of the rotor may not have a uniform inner diameter as a result of the forming or extruding of each lamination individually, which individual extrusion of laminations can introduce variances in the inner diameters of the center bores of each lamination. The variances in the inner diameter can result in a poor interference fit (or none at all) between the rotor and the crankshaft because there may be too many laminations which do not form an interference fit with the crankshaft due to the variances in the center bore. The poor interference fit between the rotor and the crankshaft as a result of the variances in the inner diameter of the center bore could result in the rotor separating from the crankshaft during operation of the motor such that the rotor would rotate with respect to the crankshaft. One way to avoid this problem is to machine a uniform inner diameter in the center bore of the rotor to ensure an appropriate interference fit with the crankshaft of the compressor. This is an additional step in the rotor fabrication process and results in additional fabrication time and expense.
Therefore, what is needed is a simple, efficient and cost-effective technique for determining if a center bore of an “as-punched” laminated rotor is acceptable for attachment to a crankshaft of a compressor without having to machine the center bore of the laminated rotor.